Setting Document Type with a Macro

I am attempting to use a Macro Type Universal Function to set the Document Type of a Purchase Order to Item, but although I can set the value in the Combobox, the document table does not update and remains as if a Service type document. Is there a trick I am missing to get this to work correctly?

In theory that should work, but in practice, after using either Focus or Click to select the ComboBox, no following input works. Using Set, or SendKeys has no impact on the screen and the ComboBox remains open awaiting user input.